The production of clean syngas and complete fuel conversion are the primary requirements for successful gasification of carbonaceous fuels for commercial applications such as production of heat, electricity, gaseous as well as liquid fuels, and chemicals. These requirements are critical to achieving desired process economics and favorable environmental impact from fuel conversion at scales ranging from small distributed- to large-scale gasification-based processes.
Among the commonly known gasifier types defined based on bed configurations (fixed bed, fluidized bed, and entrained bed) and their variants, the downdraft fixed-bed gasifier is known to produce the lowest tar in hot syngas attributed primarily to the bed configuration in which the evaporation and devolatilized or pyrolyzed products are allowed to pass through a high-temperature oxidation zone such that long-chain hydrocarbons are reduced to their short-chain constituents and these gaseous combustion and reduced-pyrolysis products react with unconverted carbon or char in the reduction zone to produce clean syngas. FIG. 1 illustrates general schematics of two variations of the downdraft gasifiers, classically known as Imbert and stratified downdraft gasifiers. The figure depicts the three primary gasification zones: evaporation and devolatilization Zone 1, oxidation Zone 2, and reduction Zone 3. The oxidizer (air) required for maintaining the high-temperature oxidation zone (Zone 2) is injected such that the location of this zone is commonly fixed.
The conversions occurring in Zone 1 are primarily endothermic, and the volatile yields are dependent on the heating rate, which is dependent on fuel particle size and temperature. The reduction reactions occurring in Zone 3 are predominantly endothermic. These reactions are a strong function of temperature and determine fuel conversion rate, thus defining fuel throughput, syngas production rate, and syngas composition.
The heat required to sustain the endothermic reactions in the reduction zone is transferred from the single oxidation zone. Thus production of clean syngas and the extent of carbon conversion heavily depend on the temperature and heat transfer from the oxidation zone to the reduction zone. As shown in FIG. 1, the temperature profile in the reduction zone sharply decreases with the increase in distance from the oxidation zone such that the reduction reaction almost freezes a few particle diameters downstream from the oxidation-reduction zone interface. As a result, this zone is termed as the dead char zone, where further conversion is completely frozen. The unconverted char is required to be removed from this zone in order to maintain continuous fuel conversion. The energy content of the fuel is thus lost in the removed char, resulting in reduced gasifier efficiency and the added disadvantage of the need for its disposal.
The critical factors of size, location, and temperature of the oxidation zone severely restrict the range of carbonaceous fuel that can be utilized in the same gasifier, which is typically designed to convert fuels with a narrow range of physicochemical characteristics, particularly particle size, chemical composition, and moisture content (e.g., typical fuel specifications for commercial biomass gasifier includes chipped wood containing less than 15% moisture and less than 5% fines). Any variation in these fuel characteristics is known to have adverse impacts on gasifier performance, and such fuels are, therefore, either preprocessed (such as moisture and fines reduction using dryer) and/or are restricted from conversion under applicable gasification technology warranty agreements.
As such, the current state of gasifier design and the inability of heretofor gasifiers to maintain a temperature profile required in gasifier zones because of the dual impact of size and temperature reduction of the critical oxidation zone, caused when fuels containing high moisture, high volatiles, or a large fraction of fine particles or fuels having low reactivity when gasified is an undesirable shortcoming of current gasifier technology. In addition, gasification of such fuels results in partial decomposition of the pyrolysis product causing undesirably high concentrations of tar in the syngas as well as adversely affecting its composition and char conversion rate, a combined effect of inadequate temperature in the kinetically controlled reduction zone. Therefore, a gasification process and/or a gasifier that can provide a long, uniform temperature zone in the gasifier, regardless of the above-referenced variations in fuel composition, would be desirable.